docs: def call_cc(fn: Callable) -> 'Cont':
          r"""call-with-current-continuation.
  
          Haskell: callCC f = Cont $ \c -> runCont (f (\a -> Cont $ \_ -> c a )) c
          """
          return Cont(lambda c: fn(lambda a: Cont(lambda _: c(a))).run(c))